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Spatial Characteristics of Lightning Relative to Supercell Kinematics and Microphysics

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Spatial Characteristics of Lightning Relative to Supercell Kinematics and Microphysics Spatial characteristics of lightning relative to supercell kinematics and microphysics Sarah M. Stough, Lawrence D. Carey Christopher J. Schultz, Daniel J. Cecil Department of Atmospheric Science Earth Science Office University of Alabama in Huntsville NASA Marshall Space Flight Center Huntsville, Alabama, United States Huntsville, Alabama, United States [email protected] Abstract— Rapid increases in total lightning flash rate, volume, updraft velocity, and measures of updraft termed lightning jumps, are often observed prior to high-impact size, such as the volume of the updraft in which convective phenomena. Relationships between lightning jumps and substantial increases in quantifiable bulk updraft properties vertical velocities are greater than or equal to -1 have been shown. However, the details that support these 10 m s [e.g., Carey and Rutledge 1996, 2000; Lang relationships are not well resolved, particularly with respect to and Rutledge 2002; Deierling et al. 2008; Deierling variability of flash rate trends in response to kinematic properties and specific, process-based connections with high- and Petersen 2008; Calhoun et al. 2013, 2014]. impact phenomena. This study addresses detail within the Meanwhile, quantified rapid increases in flash rate, complex relationships between lightning, kinematics, and referred to as lightning jumps, have been shown to microphysics via observations of a supercell thunderstorm. be related to more significant changes in the updraft Correspondence between flash rates and bulk updraft characteristics agreed with established relationships while flash of a thunderstorm [Schultz et al. 2015, 2017]. In properties varied between significant changes in flash rate. particular, quantified lightning jumps correspond Dominant flash size and altitudes of flash initiation during with changes in the 10 m s-1 updraft volume and lightning jumps corresponded spatially with a complex maximum updraft speeds that are roughly four and combination of updraft characteristics and graupel distribution. Flashes associated with a rapid decrease in flash rate showed a five times greater, respectively, than those that occur weaker spatial relationship with the updraft. in association with non-jump increases in flash rate [Schultz et al. 2017]. Keywords—supercell; Doppler radar; thunderstorm microphysics Relationships between flash rates and updraft properties have physical roots in the suggested role of the updraft in the non-inductive charging I. INTRODUCTION mechanism, storm-scale charge separation, and Observations relating rapid increases in total subsequent flash production [Reynolds et al. 1957; lightning flash rate to high-impact phenomena have Takahashi 1978; Carey and Rutledge 1996, 2000; been frequently reported in the literature, motivating Deierling and Petersen 2008]. However, the study of lightning as a metric of thunderstorm complexities within the physical underpinnings of intensity [e.g., Williams et al. 1999; Goodman et al. these relationships have not been well resolved. For 2005; Schultz et al. 2009, 2011, 2015, 2017; Darden instance, the relative roles of kinematics and et al. 2010; Gatlin and Goodman 2010]. Flash rates microphysics in the generation of a lightning jump, have generally been shown to relate to properties connections between lightning and specific such as thunderstorm graupel mass or graupel thunderstorm processes contributing to severe This work was supported by National Aeronautics and Space Administration (NASA) Severe Storms Research funding (NNH14ZDA001N), provided to 1 University of Alabama in Huntsville authors under contract from the NASA Marshall Space Flight Center (NNM11AA01A) weather, and the mechanisms behind rapid decreases 0157 UTC and 0212 UTC. No other reports of in flash rate, including those occasionally observed severe weather were documented during the analysis prior to tornadogenesis, are not well understood. In period. particular, lightning’s connection with and response In addition to providing a well-sampled case, the to vertical kinematics have not been deeply explored supercell mode represented by this storm presents with respect to downdraft-generated phenomena. the opportunity to evaluate properties of interest Investigation of these complex, interconnected within quasi-steady storm structure. Additionally, properties and processes related to high-impact the supercell storm mode includes prevalent phenomena requires more detailed analyses at finer kinematic regions for evaluation alongside well- spatial scales than reported in many studies of bulk developed microphysical fields and lightning thunderstorm properties. properties. The main updraft, forward flank The most productive path to understanding downdraft (FFD), and rear flank downdraft (RFD) details that connect lightning and thunderstorm are the primary vertical kinematic regions identified processes ultimately requires a combination of well- in supercell structure. While the RFD is thought to observed thunderstorm events and high-resolution initially result from storm flow and pressure analysis of process afforded by numerical pertubations, both the RFD and FFD are supported simulations. This study serves as an introductory by a combination of precipitation loading and latent analysis of a well-observed thunderstorm case in heat exchange as a result of microphysical which the details of flash properties, microphysics, processes [Hookings 1965; Lemon and Doswell and kinematics are considered during significant 1979; Srivastava et al. 1985, 1987; Knupp 1987, trends in lightning flash rates. Information from 1988; Vonnegut 1996; Tong et al. 1998; Naylor et available radar and lightning data is discussed, al. 2012]. where results include detailed descriptions of lightning, microphysics, and kinematic behaviors; their variation in space and time; and possible connections that may be evaluated through additional observations and future implementation of a modeling component. II. DATA AND METHODS A. 01 April 2016 Supercell Case A supercell that occurred on 01 April 2016 in northern Alabama serves as the thunderstorm of interest for this study. It was observed by a suite of instrumentation during a Verification of the Origins of Rotation in Tornadoes Experiment – Southeast (VORTEX-SE) intensive operations period, including multiple Doppler radars and the North Alabama Lightning Mapping Array (NALMA). Instrumentation platforms within the sampling domain relevant to this study are shown in Fig. 1. While data were collected within the sampling Fig. 1: VORTEX-SE sampling domain and relevant instrumentation. The domain for several hours prior to, during, and ARMOR and MAX radar are plotted as blue and red dots, respectively, while following passage of the supercell, the analysis the dual-Doppler lobes determined from 30° beam crossing are marked by the purple circles. The 10 LMA sensors that usually comprise the NALMA within period of the storm is limited to the 0100 UTC to northern Alabama and southern Tennessee are plotted as green crosses and the 0200 UTC during which time two nearby radars mobile LMA sensors added to the network for the VORTEX-SE project are plotted as blue crosses. The path of the storm of interest is plotted as a red line, were providing observations. The supercell where this analysis focuses upon the period during which the storm propagated produced an EF2 tornado that was reported between through the southern dual-Doppler lobe. 2 B. Lightning Data and Processing Methods (FID), or the number of lightning flashes that occur The NALMA typically consists of a within a given grid pixel, and flash extent density configuration of twelve sensors located in northern (FED), or the number of lightning branches ot pass Alabama, southern Tennessee, and northwestern through a given grid pixel, were gridded with Georgia that detect very high frequency (VHF) 1.0 km horizontal and 0.5 km vertical grid spacing. emissions during the propagation of lightning For gridded products and subsequent flash-based channels [Rison et al. 1999]. However, six mobile analyses, only flashes with a minimum of 10 sources LMA sensors were added to the standard NALMA were considered to reduce the likelihood of domain during the VORTEX-SE field project, including noise erroneously identified as flashes. providing additional network sensitivity. A total of Processed lightning flash data were associated 16 of the 18 available sensors were active during the with the supercell according to the methods analysis period of this supercell, all located within described in Stough et al. [2017]. Briefly, using the northern Alabama and southern Tennessee [NASA Warning Decision Support System – Integrated 2001]. VHF sources detected by LMA sensors are Information (WDSS-II) software, storm objects located in time and space utilizing a time of arrival were identified using environmental and radar technique [Thomas et al. 2004]. While data from a reflectivity data [Lakshmanan et al. 2007]. Storm minimum of four sensors are required to resolve the boundaries based on these objects were used to time and location of a source, six or more sensors isolate lightning flashes associated with the storm of are typically utilized in practice to reduce the interest. Flashes collected via storm tracking were inclusion of noise. Source data are subject to binned in 2-min intervals and processed to location errors on the order of 1 km in the vertical determine 1-min flash rate and the presence of any and 500 m in the horizontal within
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